How do I utilize the individual outputs of the Apollo within Ableton?

I apologize if this has been covered before. I tried searching through several forums and could find nothing.

I have a small project studio recording my own analog synthesizers and guitar.I record into my UAD Apollo Duo (which I love) onto disk via Ableton Live.

I really want to use some of my outboard effects and analog filters/fx on my synths, but I can't figure out how to route to the individual outputs.

They're selected in Live, but I'll send a drum loop out to, say Output 3 for my Modular to process, but no signal appears at the output. I've tried listening directly to the output, and cycling through all available outputs in Live, but I never hear it actually come out. (I've read that DAWs sometimes mis-label the outputs on the Apollo so I've listened to Output three, while selecting output 3, output 4, etc...).

Can anyone walk me through how to utilize individual outputs on the Apollo?